1. Technical Field
The present invention relates to an optical function device using a PLZT waveguide, for example, a waveguide type optical amplifier that has an amplifying function that amplifies light transmitted by optical fiber without optical-electric conversion, and a fabrication method thereof.
2. Description of the Related Art
In optical communications networks, efforts are continuing to develop, from point to point optical communication connecting individual inter-nodes, optical communication carrying out Add-Drop Multiplexing between points, and also optical communication connecting plural inter-nodes just with an optical signal, without converting to an electrical signal. Also, since volumes of traffic and functionality of optical communications networks are increasing, multiplexing of plural wavelengths in a single strand of optical fiber, and, the opposite thereof, of dividing optical signals of plural wavelengths being transmitted in a single strand of optical fiber into their individual wavelengths (WDM: Wavelength Division Multiplexing), have been put into practice.
In these methods, it is necessary to transmit optical signals with different wavelengths from each other in a single strand of optical fiber, and to carry out intermediate relay amplifying according to the transmitting distance without converting into electrical signals. Optical amplifiers, for carrying out long haul transmissions without conversion from optical into electrical signals, support optical networks.
As optical amplifiers, optical fiber amplifiers with optical amplification media of optical fibers in which a rare earth element has been added to the core, for example Er (erbium) Doped Optical Fiber Amplifiers (EDFA), have been put into practice, and application of such amplifiers to optical communications is continuing to progress at a fast pace. Such EDFAs are operated in the 1.55 μm wavelength band where the loss in silicon optical fibers is at a minimum, and are known for their superior characteristics of high gain of 30 dB or more, low noise, wide gain band, lack of polarization dependence in the gain, high saturation power output, and the like.
However, EDFAs are optical fibers of around 10 m in length, and have the problem that it is difficult to make the devices themselves small. Therefore, for the future, effort is being put into optical amplifiers which include laser light sources for excitation, miniaturization of optical amplifiers, and also into integration and standardization of plural optical amplifier units, and modulization of high-specification devices with modulators, switches, wavedividers and the like integrated onto a single substrate to reduce the size, with development progressing in optical amplifiers in waveguide form using, optical waveguides, materials with rare earth elements added that can be used as amplification media at the desired wavelength band.
An optical switch is one of the most important components, and is a component that, for example, is used for switching between plural optical fibers according to demand, and used for switching to secure a diversion route when there is damage to a network. Optical waveguide switches that are superior in being miniaturized are generally formed as channel optical waveguides in LiNbO3, semiconductor compounds, quartz, or polymers, and are provided with an optical switch for electrically controlling the light progress direction at the intersection portions of each of the paths, or with an optical gate for electrically controlling, open or close, the progress of the light.
Optical switches using quartz or a polymer, are made with a core size that is about the same size as the mode field diameter, and have the characteristic that the insertion loss is low because the optical coupling efficiency from the optical fiber is good. However, there is the problem that, by running current through a heater provided on the surface of the optical waveguide, in order to change the direction of light progression using a change in refractive index due to the thermo-optical effect, the reaction time of such optical switches is slow. Furthermore, in order to use such a heating method with a heater, several hundreds of mW of power is consumed for the single electrode, and there is the problem that fields of use are limited.
Other than these, there are optical waveguide optical switches that use organic nonlinear optical materials. By a structure of an optical waveguide of a field poled polymer or the like, sandwiched between upper and lower electrodes, an optical switch that can be driven at a low voltage can be configured, but field poled polymers have the problem of temperature stability when compared to ferroelectric oxide materials, and, in reality, are not readily applicable.
In the case of optical waveguide optical switches using compound semiconductors and quantum wells, increasing speeds is possible, and there is the expectation of reducing the driving voltage since voltage can be applied above and below the optical waveguide core. However, there is the problem that the insertion loss is high because the optical coupling efficiency from the optical fiber is poor due to the small core size, and effort is being put into various areas. As well as this, there is the problem that the switching characteristics are inferior due to the occurrence of light absorption when switching by applying an electric field, and there are problems such as, since wafer size is limited, it is difficult to configure large scale matrixes of optical switches.
The most typically used materials for optical switches are ferroelectric oxide materials, and in the case of one of them, LiNbO3, if voltage is applied to electrodes of an optical switch then, due to the electro-optical effect, there is a change in the refractive index, and by this the conditions of the light can be changed at high speed, and depending on the set conditions, the progression direction of the light is changed. Because of this, in an optical switch it is possible to selectively output light that entered from two input terminals to two respective output terminals. Optical switches using LiNbO3 may be produced by making a waveguide on a single crystal wafer by diffusion of Ti or proton exchange, and the core size can be made to be about the same as the optical fiber mode diameter, therefore, since the optical coupling efficiency is good, insertion loss is small, and workable optical switches are known.
However, since it is a configuration in which coplanar electrodes are disposed on the optical waveguide faces and voltage is applied, when the distance between electrodes becomes large and the field profile also does not become optimal, and in order to have no polarization depencence present the driving voltage becomes high, at 40 volts, and so that the driving voltage does not become even more extremely high, usually a long electrode of 7 mm or more is required. Further, in order to make a waveguide to a single crystal wafer by diffusion of Ti or proton exchange, it is not possible to make the effective refractive index of the channel optical waveguide high enough compared to the refractive index of the surroundings, and not possible to make the difference in refractive index high. Due to this, the need arises to make the radius of curvature of the channel optical waveguide as big as 50 mm, and, in the example of an 8×8 optical switch matrix, the size of becomes about 70 mm.
As above, whichever of LiNbO3, compound semiconductors, quartz or polymers are used, it is not possible to obtain an optical waveguide matrix optical switch which satisfies at the same time all of the characteristics of optical switch size, driving voltage (or driving current or power consumption), switching speed, cross-talk, insertion loss, and temperature stability.
As a material for solving these problems, PLZT, that is Pb1−xLax(ZryTi1−y)1−x/4O3(PLZT: 0<x<0.3, 0<y<1.0), is attracting attention for optical waveguides, and optical switches are in the process of being developed with high speed, low driving voltage, low power consumption, small size.
However, regarding PLZT ceramics, there is information about investigations into the photoluminescence characteristics thereof, according to Ballato et al. (J. Luminescence, 86 (2000) p.p. 101-105), but this does not include investigations into PLZT waveguide optical amplifiers. Therefore, the appropriate doping amounts and doping methods relating to rare earth element-doping were not known, and configuring an optical amplifier was difficult.
That is to say, with the aim of raising the amplification efficiency and making optical amplifiers of smaller size, or increasing the width of the amplification wavelength band, it is necessary to increase the concentration of rare earth ions, for example Er3+ ions added per unit volume, but generally, when the concentration is increased, a condition occurs in which multiple Er3+ ions exist in clusters, and this is an impediment to increasing the amplification efficiency.
Therefore, for investigations into the optical amplifiers with PLZT waveguides as the medium, raising the amplification efficiency and also increasing the width of the amplification wavelength band, when increasing the concentration of rare earth ions, for example Er3+ ions, added into a PLZT waveguide layer (core layer), it is necessary to consider the optimum concentration that can suppress clusterization of the added Er3+ ions, and also to consider methods of forming an Er-doped PLZT film optical waveguide. In quartz and Al2O3 waveguides, it is possible to form an optical amplifier component using film forming methods such as chemical vapor deposition (CVD) methods, flame hydrolysis deposition (FHD) methods, sputtering methods, vapor deposition methods, and the like, and adding a rare earth to the raw material gases, sputtering targets, or vapor sources.
However, if the rare earth is added at a certain concentration or above, in whichever of the film forming methods, defects develop such as precipitation out, and the addition amount of the rare earth species becomes about 1 mol %. For example, in the Er-doped Al2O3 waveguide optical amplifier formed by sputtering and reported by Musa et al. (IEEE J. Quantum Electronics, Vol. 36, No. 9 (2000) p.p. 1089-1097) doping was carried out up to a concentration of 0.74 mol %, and a net gain of 1.0 dB/cm was reported. However, since a concentration of such a level is not able to obtain sufficient optical amplifying efficiency, waveguides for optical amplifier use must be elongated.
Furthermore, when investigating PLZT waveguide optical amplifiers, it is necessary to achieve a state of containment of more of the internally amplified light within the optical waveguide layer (core layer), and also necessary to reduce the overall loss. Specifically, it is necessary to achieve conditions of the waveguide in which a predetermined difference in refractive index between the core and the surrounding cladding is achieved.
There is a need for an optical amplifier including a PLZT optical waveguide layer with added rare earth element, the optical amplifier being one of small size and high efficiency, and a fabrication method for the same is also needed.